1. Field of the Invention
The invention relates to a mask substrate used in a lithographic process for manufacturing, e.g., semiconductor devices or liquid crystal display devices, and to a projection exposure apparatus that transfers a circuit pattern onto a photosensitive substrate, such as, e.g., a semiconductor wafer or a glass plate, from a mask substrate. The invention also relates to a method of forming a circuit pattern onto a photosensitive substrate using a projection exposure apparatus.
2. Description of Related Art
In manufacturing semiconductor devices (e.g., VLSI) or liquid crystal devices, it is indispensable to perform a lithographic process for exposing and transferring a circuit pattern onto a photosensitive substrate, such as a photoresist-coated semiconductor wafer or glass plate. The lithographic process uses a projection exposure apparatus to project and expose a circuit pattern image formed on a mask substrate (also referred to as a reticle) onto a photosensitive substrate through a projection optical system having a magnification equal to or less than 1. This projection exposure apparatus is called a stepper, because it operates in a step-and-repeat manner to repeat an action of driving a two-dimensional driving stage supporting a photosensitive substrate by a predetermined amount to a next shot area for exposure, every time exposure of the projection image of the reticle circuit pattern onto a shot area on the photosensitive substrate has been accomplished.
In order to simultaneously achieve high resolution and a broader exposure field, a step-and-scan type projection exposure apparatus has recently been proposed. In this technique, the reticle and the photosensitive substrate are scanned one-dimensionally relative to the field of view of the projection optical system during exposure-of one shot. When exposure is not performed, the photosensitive substrate is driven in a stepwise manner. See, e.g., "Optical/Laser Microlithography II", SPIE Vol. 1088, at 424-433 (1989). Also see, e.g., U.S. Pat. No. 5,477,304 and U.S. Pat. No. 4,924,257, both of which are incorporated herein by reference in their entireties.
The reticle is fixed to and supported on the reticle stage in the apparatus by applying a vacuum thereto, so that the main surface of the reticle, on which the circuit pattern is formed, is precisely aligned with the object surface of the projection optical system. By .illuminating the pattern area (normally, a rectangular area of illumination is used, although, as disclosed in the above-incorporated patents, the illumination area also can be arcuate or hexagonal, for example) of the reticle mounted on the stage with exposure illumination light, the pattern image formed on the pattern area is projected through the projection optical system and onto the photosensitive substrate.
Generally, the reticle is formed by etching a light-blocking material (e.g., chromium) layer formed on the main surface of a quartz plate by evaporation into the circuit pattern. Alternatively, a certain type of phase shift reticle is formed by etching a transmissive material shifter layer formed on the main surface of the quartz plate into the circuit pattern.
The size of the reticle has increased from 4 inches, to 5 inches, and currently to 6 inches as a standard, as integration techniques have improved and as the device (i.e., the chip) size has increased. In an exposure apparatus for producing liquid crystal devices in which the circuit pattern is exposed by a scanning scheme using a projection optical system with a magnification of 1, the mask and the photosensitive substrate (glass plate) are the same in size, and a mask larger than 40.times.40 cm may be used.
In the manufacturing site of semiconductor devices, mass production of 64M D-RAM has started. Moreover, although still in the trial stage of manufacturing, a great deal of study and development for mass production of 256M D-RAM and 1 G D-RAM has been made. It is expected that mass production of both 256M (Megabyte) memory and 1 G (Gigabyte) memory devices will require a projection exposure apparatus that uses an ultraviolet light source.
On the other hand, the accuracy standard has become more and more strict in various functions for manufacturing such devices required in the projection exposure apparatus. In particular, very strict precision in image formation of the projection optical system and alignment accuracy between the reticle and the photosensitive substrate (wafer) are required. For this reason, a design and manufacturing method for approaching an ideal image formation capacity of the projection optical system has been desired. Developing various types of sensors is also desired to improve the alignment precision.
However, even though efficiency and performance are improved in the projection exposure apparatus, a problem of heat energy accumulation still remains, which is caused by continuously illuminating the reticle with illumination light. The extent of heat accumulation varies depending on the material used to make the circuit pattern formed on the reticle and the light transmittance through the reticle. If the circuit pattern is formed with a non-transmissive material layer, and if the ratio of the total light-blocking portion to the overall illuminated area is greater, heat accumulation becomes great.
Due to such heat accumulation, the temperature of the reticle rises, and as a result, the reticle slightly expands. Since the four corners of the reticle are fixed to the reticle stage at the vacuum mounting positions, the reticle is physically warped by the heat expansion. This adversely affects the flatness of the reticle. If the flatness of the reticle is degraded, the projected pattern image will contain errors, such as distortion aberration, image surface distortion, or image surface tilting, even if the imaging property of the projection optical system is close to ideal. This error adversely affects the final image of the circuit pattern to be transferred onto the photosensitive substrate.
In particular, a problem arises from the fact that heat accumulation and heat release of the reticle slightly varies depending on the change in the exposure sequence of the photosensitive substrate and the illumination conditions. More seriously, a reticle may be replaced with another reticle having a completely different circuit pattern, and tendencies of heat accumulation and heat release are different for each reticle. Thus, although the same projection exposure apparatus is used, the quality of the image to be transferred onto the photosensitive substrate changes depending on the circuit pattern.
Even in a single reticle, the influence of heat accumulation is totally different, for example, between a situation where the reticle has just been mounted on the exposure apparatus, and a situation where exposure operation has been continuously performed. Consequently, the quality of the transferred pattern image deteriorates gradually.